1. Field of the Invention
This invention relates to engraving devices, and more particularly to a method and apparatus for detecting surface irregularities and for correct adjustment of engraving in response thereto.
2. Description of the Related Art
Prior art devices of the type shown in U.S. Pat. Nos. 4,450,486; 5,424,846; 5,438,422; 5,424,845; 5,329,215; 5,652,659 typically comprise an engraving head having an engraving device, such as a diamond stylus, and a guide shoe. The guide shoe bore against a surface of a cylinder and provided a reference for the engraving process. An electromagnetic driver mounted within the engraving head caused the engraving device to oscillate into engraving contact with the cylinder as the cylinder rotated about its cylindrical axis, thereby causing either a helical or cylindrical tract of engraved areas or cells to be engraved on the surface of the cylinder.
The cylinders engraved oftentimes had surface irregularities, such as indentations or “bumps” or other artifacts that appeared on the surface of the cylinder. In engraving heads of the prior art, the engraving head had a sliding shoe mount assembly that was very stiff and forced the entire engraving head to follow the surface of the cylinder. The goal of the engraving process is to cut diamond-shaped cells into the surface of a copper cylinder that will be used for gravure printing. The depth of the holes or cells must be controlled with an error less than a fraction of a micron (micro meter). This control must take place while the surface of the cylinder moves radially by hundreds of microns. By having the entire head follow the surface of the cylinder, a cutting diamond stylus is provided with a local reference as to where the cylinder surface is so that it can accurately cut to depth.
A present shoe mount assembly is provided in the engraving machine model number 850-GS-XX available from Max Daetwyler Corporation, the assignee of the present invention. The head has a brass finger about two inches long that flexes in a radial (cylinder radial) direction under the force of a screw. The finger is mounted to the engraving head casting at the bottom and the sliding shoe mounts to the top end of the finger. The top of the finger is supported radially (again with respect to the cylinder) from behind by a fine-threaded screw. The screw adjusts the position of the shoe with respect to the engraving head casting and provides a stiff support from between the shoe and the casting. The result is a stiff, but adjustable, support for the sliding shoe. The effective mass of the engraving head is, in a typical engraver, approximately six kilograms.
In other engraving systems, such as systems provided by Rudolph Hell Company, the engraving head and the shoe diamond is mounted to the tip of a screw threaded into the casting of the engraving head. The axis of the shoe screw is oriented radially with respect to the cylinder surface. Rotating the shoe screw adjusts the relative position of the engraving head casting and thus the position of the cutting diamond relative to the surface of the cylinder. The effective mass of these types of engraving heads is on the order of about two kilograms.
The gravure industry has changed recently and where the surface of the cylinder to be engraved could be seen to be nearly perfect, many customers now want to use much rougher cylinder surfaces. With rougher surfaces, more force is applied to the sliding shoe while following the cylinder surface and the force shows up as change in depth of the engraving. The engraving head and the carriage on which it is mounted have mechanical vibrations that can be excited by the shoe dragging on the cylinder surface. Vibration modes can be excited both radially and tangentially to the cylinder. If a lightly dampened vibration mode is driven by a cylinder surface ripple that happens to fall at the vibration resonance, the resulting resonance vibration buildup can be larger than the original surface ripple. All of this causes the size and/or shape of the engraved cells to be inaccurate.
What is needed, therefore, is a method and apparatus for improving engraving and overcoming the problems associated with surface irregularities.